Reactions of Monosaccharides

Reactions of MonosaccharidesReactions of Monosaccharides

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Oxidation of sugarsOxidation of sugars• Oxidation with CuOxidation with Cu++ The Benedict’s test The Benedict’s test• Sugars that are capable of producing a positve Benedicts Sugars that are capable of producing a positve Benedicts

test are called test are called reducing sugarsreducing sugars..

Oxidation of AldehydesOxidation of Aldehydes

Aldehydes can be oxidized Aldehydes can be oxidized with the copper(II) ion (Cuwith the copper(II) ion (Cu2+2+))

• The CuThe Cu2+2+ ion forms a clear ion forms a clear blue solutionblue solution

• The CuThe Cu++ that is produced in that is produced in the reaction forms an the reaction forms an orange/red precipitate.orange/red precipitate.

• The reaction is called the The reaction is called the Benedict’s reaction, and Benedict’s reaction, and has been used for years has been used for years in a clinical setting to test in a clinical setting to test for the presence of for the presence of glucose in the urine.glucose in the urine.

C

O H

C

C

C

C

CH2 OH

H

H

HO

H OH

H

OH

OH

glucose

CuCu2+2+CuCu2+2+ CuCu++CuCu++CuCu2+2+ + + CuCu++

CuCu2+2+ + + CuCu++


Oxidation of sugarsOxidation of sugars• Even though ketones should not give a positive Benedict’s Even though ketones should not give a positive Benedict’s

test, ketoses do.test, ketoses do.• This is because under the basic conditions of the test, the This is because under the basic conditions of the test, the

ketoses can isomerize to form aldoses, which react.ketoses can isomerize to form aldoses, which react.• Benedict’s test has been replaced with a more Benedict’s test has been replaced with a more

specific, enzyme catalyzed, glucose oxidase test.specific, enzyme catalyzed, glucose oxidase test.


Reactions with alcohols to form hemiacetals and hemiketalsReactions with alcohols to form hemiacetals and hemiketals• Since monosaccharides contain both hydroxyl groups Since monosaccharides contain both hydroxyl groups

along with either aldehyde or ketone groups, they can react along with either aldehyde or ketone groups, they can react to form hemiacetals and hemiketals.to form hemiacetals and hemiketals.

Reactions of Alcohols with Aldehydes and Ketones


The first reaction, which is similar to the reduction of The first reaction, which is similar to the reduction of aldehydes and ketones, involves adding an alcohol across aldehydes and ketones, involves adding an alcohol across the carbonyl to form a hemiacetal (from aldehydes) or a the carbonyl to form a hemiacetal (from aldehydes) or a hemiketal (from ketones).hemiketal (from ketones).

CH2H3C

Propanal(Aldehyde)

C

O

H

O CH2 CH3

H

+

Ethanol(Alcohol)

CH2H3C C

O

H

O CH2 CH3

H

(Hemiacetal)

CH3

Propanone(Ketone)

C

O

CH3

O CH2 CH3

H

+

Ethanol(Alcohol)

CH3 C

O

CH3

O CH2 CH3

H

(Hemiketal)



In carbohydrates, the carbonyl group and the alcohol that In carbohydrates, the carbonyl group and the alcohol that react, can come from the same molecule.react, can come from the same molecule.• This will produce a ring molecule.This will produce a ring molecule.

Cyclic Form of MonosaccharidesCyclic Form of Monosaccharides

Monosaccharides form rings by reacting one of the hydroxyls Monosaccharides form rings by reacting one of the hydroxyls with the carbonyl to form a hemiacetal or hemiketal:with the carbonyl to form a hemiacetal or hemiketal:


Usually these are drawn using a Usually these are drawn using a Haworth projectHaworth project::

• The OH’s that were on the The OH’s that were on the rightright in the Fisher projection are in the Fisher projection are placed in the placed in the downdown position on the Haworth projection position on the Haworth projection

• The OH’s that were on the The OH’s that were on the leftleft in the Fisher projection are in the Fisher projection are placed in the placed in the upup position on the Haworth projection position on the Haworth projection

• The CHThe CH22OH on the number 5 carbon points up for D sugars OH on the number 5 carbon points up for D sugars and down for L sugars. and down for L sugars.


The hemiacetal or hemiketal carbon that forms in the ring is The hemiacetal or hemiketal carbon that forms in the ring is called the called the anomericanomeric carbon. carbon.• The anomeric carbon is also chiral, which increases the The anomeric carbon is also chiral, which increases the

number of chiral carbons by 1 and increases the number of number of chiral carbons by 1 and increases the number of stereoisomers.stereoisomers.

• The two forms of the anomeric carbon are designated as α The two forms of the anomeric carbon are designated as α or β.or β.• The α-anomer has the -OH pointing down in the ring form.The α-anomer has the -OH pointing down in the ring form.• The β-anomer has the -OH pointing up in the ring form.The β-anomer has the -OH pointing up in the ring form.

OCH2OH

OHOH

OH

OH

OCH2OH

OHOH

OH

OH


When naming the ring forms of monosaccharides, the When naming the ring forms of monosaccharides, the endings -endings -pyranosepyranose and - and -furanosefuranose to designate the six- to designate the six-member and five-member rings, respectively.member and five-member rings, respectively.

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In general, the -OH on the chiral carbon furthest from the the In general, the -OH on the chiral carbon furthest from the the carbonyl is the one that reacts to from the pyranose or carbonyl is the one that reacts to from the pyranose or furanose ring. This means thatfuranose ring. This means that• Aldohexoses will form pyranose rings:Aldohexoses will form pyranose rings:

• Aldopentoses and ketohexoses will form furanose rings:Aldopentoses and ketohexoses will form furanose rings:

DD-glucose-glucose(aldohexose)(aldohexose)

DD-glucose-glucose(aldohexose)(aldohexose)

DD-ribose-ribose(aldopentose)(aldopentose)

DD-ribose-ribose(aldopentose)(aldopentose)

DD-fructose-fructose(ketohexose)(ketohexose)

DD-fructose-fructose(ketohexose)(ketohexose)

إن الله ومالئكتــه يصلــون على النبي يــأيــها الذيـن آمنوا صلــوا عليه وسلمـــوا تسليمـــا

إن الله ومالئكتــه يصلــون على النبي يــأيــها الذيـن آمنوا صلــوا عليه وسلمـــوا تسليمـــا

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Reactions of Monosaccharides